Light-emitting diodes, or LEDs in short, are gradually taking over all areas of everyday life. Lighting systems, for example streetlights, vehicle lights, or interior lights, which were equipped in the past with conventional incandescent lamps, are being replaced by or refitted with LED lamps more and more often. This is also true for lighting systems which are used in (mobile or grid-connected stationary) medical diagnostic devices, for example, microscopes, otoscopes, or endoscopes. Incandescent lamps or subminiature incandescent lamps in medical diagnostic devices are gradually being replaced by LED lamps.
The advantage of LED lamps in relation to conventional incandescent lamps is obvious. For example, an LED lamp only consumes a fraction of the electrical energy which a conventional incandescent lamp consumes to generate a comparable luminous flux (and therefore a comparable brightness). Furthermore, in LED lamps, color, color temperature, and brightness can be adjusted flexibly depending on the activation. The service life of LED lamps is, at tens of thousands to 100,000 hours, higher by one or more orders of magnitude than that of conventional incandescent lamps, in which service lives of 1000 to 2000 hours can be expected. In the field of subminiature incandescent lamps for medical diagnostic devices, service lives of even only 10 to 100 hours are achieved. Furthermore, the developers have a substantially higher amount of free space in the optical and mechanical design of LED lamps than in previous solutions.
Since conventional incandescent lamps represent a simple ohmic consumer having a linear current-voltage characteristic curve, the operation of incandescent lamps is very simple. The incandescent lamp merely has to be electrically coupled to a voltage source. The supply voltage provided by the voltage source causes a current proportional to the applied supply voltage to flow through the incandescent coil of the incandescent lamp, and this current causes the incandescent coil to illuminate. The proportionality between the applied supply voltage and the current is determined by the resistance of the incandescent coil according to Ohm's law. By changing the applied supply voltage, the brightness of the incandescent lamp (i.e. the luminous flux of the incandescent lamp) can be changed. The brightness (or dimming) of incandescent lamps can therefore be adjusted directly via adjustment of the supply voltage.
In contrast, the current-voltage characteristic curve of an LED has a diode characteristic having an exponential curve. Above a certain voltage (LED operating voltage), the LED current rises steeply, while the voltage remains almost unchanged. The brightness of an LED lamp therefore cannot be regulated directly via the provided supply voltage. Rather, LEDs are regulated by adjusting the LED current.
Electronic circuits (so-called LED drivers) are used for adjusting the LED current. Conventional LED drivers are designed to provide an approximately constant LED current for operating the LEDs. Circuits which keep the LED current constant or compensate for a voltage difference between the power supply and LED are known, inter alia, from U.S. Pat. No. 7,276,025 B2, EP 2 319 391 A1, US 2008/297069 A1, and U.S. Pat. No. 7,459,959 B2.
The dimming of an LED lamp can be achieved, for example, by changing the LED current. LED drivers, which in comparison to conventional LED drivers have an additional control channel having an external control input to which a control voltage is applied for adjusting the LED current, are also known from the prior art. In most cases, an additional analog signal (for example, 0-10 V) or a signal having pulse width modulation (PWM) is applied to the control terminal of the driver. Control options via the feedback path of LED drivers are also known.
An LED circuit is known from U.S. Pat. No. 8,786,210 B2, in which the LED is electrically coupled directly to the supply voltage input of the LED driver via a measurement resistor having a voltage output of an LED driver and via two further resistors, which function as voltage dividers. The resistors functioning as the voltage dividers have the effect that the potential difference (voltage) applied to the measurement resistor can be changed according to the supply voltage. Since the LED driver adjusts the LED current according to the potential difference, the LED current (and therefore the brightness of the LED) can be adjusted by changing the supply voltage.
In mobile and stationary diagnostic devices, light microscopes and instrument lights, lighting systems are generally used, which enable dimming of the incandescent lamp over a predefined brightness range. Either electronically variable voltage sources or fixed voltage sources (for example, accumulator cells), which are electrically coupled to an adjustable electrical resistor (for example, a rheostat), are used for dimming the incandescent lamp in the lighting systems. The adjustable electrical resistor is arranged in series with the incandescent lamp in the electrical circuit. Because of its serial arrangement, the adjustable resistor is also referred to hereafter as an adjustable series resistor. It functions as an adjustable series resistor for the continuous adjustment of the supply voltage for the incandescent lamp. If the resistance value at the adjustable resistor is increased, the supply voltage applied at the incandescent lamp decreases and vice versa. The brightness of the incandescent lamp increases or decreases accordingly. In other words, the adjustable series resistor functions as a brightness regulator.
This simple regulating mechanism no longer functions if the incandescent lamp of a lighting apparatus is replaced by an LED lamp having a separate LED driver. The reason for this is that LED lamps have a substantially higher resistance than incandescent lamps having equal or similar luminous flux. Therefore, the voltage division ratio specified by the series resistance and the resistance of the LED lamp changes such that a change in the series resistance only causes a slight change in the supply voltage. Therefore, even if an LED driver having an external control unit is used for the voltage-dependent control of the LED current, only slight, hardly perceptible dimming can be achieved. In particular, the fine adjusting in the lower brightness range, which is important for medical applications, is lost.
The use of a rheostat, which is designed for adjusting higher resistance values, could technically solve the problem, but would have the result that, upon the replacement of the incandescent lamp by an LED lamp, the provided electrical supply apparatuses having a fixed voltage source and a rheostat would also have to be completely replaced. In medical diagnostic devices, the fixed voltage source and rheostat are generally integrated in the instrument handle. Such instrument handles are used in the millions worldwide. The replacement of the instrument handles would cause very high costs and would be logistically very complex, which is not economically acceptable.